Environment variables editor: dump editable combobox, use extra name editor

Summary:
Also:

• use consistently term "profile" instead of "group"
• mark default profile with suffix "(default profile)" in combobox list
• make buttons for editing profiles icon-only, similar to other places with buttons in one row with the combobox (avoids also size issues with translated button texts)
• splits the single model class into two separate model classes, one for list of profiles, one for variables of a profile
• makes all UI updates (like button states) follow model signals instead of direct setting

Reviewers: KDevelop, apol, kfunk

Reviewed By: KDevelop, apol, kfunk

Subscribers: kfunk, apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4730